The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the accurate generation of gating signals for use in cardiac gated MR imaging and spectroscopy.
The data required to reconstruct an MR image is acquired by an MRI system over a period of time. In most acquisitions this time period extends over many cardiac cycles of the patient and sometimes it is necessary to synchronize the acquisition with the cardiac cycle. This is accomplished by monitoring an ECG signal produced by the patient's heart and triggering, or gating, the data acquisition sequence when the R-peak in the QRS complex is detected.
The accurate detection of the R-peak in the ECG signal is very difficult in an MRI system environment. First, the quality of the ECG signal itself is seriously degraded by the magnetic induction effects caused by the strong magnetic fields used in MRI systems. Significant inductive noise is added to the ECG signal by patient movement and blood flow as well as "gradient noise" produced by the rapidly changing magnetic field gradients used during all MRI acquisitions.
When used for gating MR data acquisition, the detection and signaling of the R-peak event in the ECG signal must be done on a real-time basis. In a cardiac gated MRI scan, data is acquired over a portion of the cardiac cycle following each ECG gating signal, and it is a major objective to begin acquiring data as soon as possible after the occurrence of the R-peak. Since this acquisition window begins immediately after the QRS complex in the ECG signal, any time delay in producing the gating signal translates to a corresponding reduction in the data acquisition window. Such delays prevent the acquisition of images which depict the heart in the early systolic phase of the cardiac cycle. A delay in excess of 30 milliseconds is unacceptable for many scan types.
The need for a real-time gating signal means that very limited filtering of the noisy ECG signal can be performed. For example, a filter designed to block frequencies above 15 Hz can impose a time delay on the ECG signal of greater than 30 milliseconds. Since this is excessive for MR gating purposes, the R-peak detector must function accurately and consistently with a relatively noisy ECG signal.
R-wave detectors used in the past generally fall into three classes. The first employs a band pass filter and is based on the principle that the QRS complex is rich in 10 to 17 Hz frequency components and that the ECG waveform can be passed through a filter which has a center frequency of about 10 Hz so that the accentuated frequency can be detected. One problem with this approach is that the ECG signal is delayed too long by the filter as discussed above. Another problem with this class of detectors is that patient movement and gradient induced noise may contain components with about the same frequency range so it is difficult for the detector to distinguish them from a true R-wave or QRS complex. Moreover, the QRS portion of the ECG waveform with certain types of heart defects is much wider than the normal or average width for a healthy subject so it is also rich in frequencies lower than the center frequency of the filter which is set for the normal QRS complex.
Another class of R-wave detectors operates on the principle that the slope of the leading and trailing edges of the QRS complex are uniquely different from those of the P and T wave portions of the ECG. The assumption is, therefore, that the derivative of the ECG waveform can be obtained and that when the output exceeds some preset threshold value, the equivalent of some preset slope, that this can be detected. The disadvantage of prior derivative class detectors is that some technique must be used to limit the noise induced by the gradient amplifiers. These gradient induced noise spikes have slopes equal to or greater than that of the QRS complex. Such similar slopes are hard to distinguish from the R-wave slopes. In U.S. Pat. No. 3,939,824 this problem is addressed by requiring that the derivative, or slope, of the ECG signal be maintained above the preset threshold value for a minimum time interval.
A third approach is an amplitude based technique that relies on analyzing the peaks in the ECG signal and setting an amplitude threshold that, when reached, will produce the gating signal. The accuracy of such techniques is highly correlated with the amount of filtering that is used, since high amplitude noise spikes can trigger the detector. In a current system used by the General Electric Company in its MRI systems, a measurement accuracy of 95.4% (i.e. a failure rate of 1 out of 22 R-peaks) is achieved with an amplitude based method that imposes a time delay of approximately 25 milliseconds on the gating signal.